Color-Correct Images with Photoshop’s Curves Tool

Digital cameras keep getting smarter, but it seems like every day we receive a friend’s photo in which the color is way off. Any schmo can click the auto correction button in iPhoto or Photoshop, but those results aren’t always the most desirable. Photoshop CS3’s Curves dialog offers a way to manually fix color casts, so you can have a reason to brag about the finished image.

This image from 2003 shows the photographer’s inexperience—the focus is soft in the lower part of the frame, and the camera couldn’t find a white balance.

After correcting the white, the image looks a lot more like it ought to.

What you need: Adobe Photoshop CS3 ($649 à la carte, or with a Creative Suite bundle, www.adobe.com) and a digital photo with an objectionable color cast, containing something you know should be white or gray.

1. Stay InformedThe values for red, green, and blue are way off on the chopstick wrapper that we know should be white, so this image definitely needs correction. Another benefit to using the Info palette is knowing the actual values of the colors, which is handy if your monitor isn’t calibrated. Click image to embiggen

Correcting color casts is sort of like landing a plane: It’s possible to do it without looking at flight instruments, but why would you want to? First select Window > Info to reveal Photoshop’s Info palette, which displays information about where the cursor is pointed in an image. The palette contains two eyedropper icons, each with right-pointing triangles you can click. Click them and select RGB color for one and Grayscale for the other. Select 8-bit (0-255) under the RGB eyedropper. Now the Info palette will show you the R, G, and B values for red, green, and blue, the primary colors of light, as well as the K value, which is the percentage of black that would be used if the image were converted to grayscale.

It’s also smart to use cursors that make it obvious where they are pointing. Select Photoshop > Preferences > Cursors. Under Painting Cursors, click Full Size Brush Tip and check the box for Show Crosshair in Brush Tip. Under Other Cursors, click Precise. Click OK to keep the changes.

Finally, make sure the Eyedropper tool isn’t so precise that it samples just one pixel at a time. Select the Eyedropper tool from the toolbox on the left of the window, and its options will appear at the top left of the workspace. Set the Sample Size to 3 By 3 Average (or larger) to prevent one rebellious pixel from ruining the party.

2. Analyze the Image The values for red, green, and blue are way off on the chopstick wrapper that we know should be white, so this image definitely needs correction. Another benefit to using the Info palette is knowing the actual values of the colors, which is handy if your monitor isn’t calibrated. Click to embiggen image

Open your JPEG in Photoshop. Move the cursor over parts of the image, and watch the Info palette to get an idea of how the colors work together to make the image. The Info palette will display the red, green, and blue values of the sample area, with 0 being the darkest and 255 being the lightest. The farther apart these numbers are, the less neutral a color will be. You can also look at the K value, with 0% being white and 100% being black, to learn about the lightness or darkness of an area.

Find something you know to be a relatively bright neutral color in real life and mouse over it. You’re after something bright white or gray, but not a highlight, which will probably already be pure white. Look for something with a K percentage of greater than 5, but less than 40 or so. Mouse over it, and look at the red, green, and blue values. If the image has an objectionable cast, the values of at least one of the RGB channels will probably be more than 10 in either direction from the other two.